Crossover is a potentially serious threat to a welfare reform evaluation, because it blurs the distinction between experimental and control cases and can lead to biased estimates of the impacts from welfare reform. The terms and conditions of Section 1115 welfare waivers have devoted considerable attention to minimizing the risk of crossover, and we recommend that states seek to adhere to the waiver standards for administering experimental or control policies to cases that migrate, merge, or split.
States can reduce the incidence of crossover in welfare reform evaluations in at least three additional ways:
- A large portion of the state's welfare population could be included in the research sample. This measure, while costly, would reduce the likelihood of crossover through migration to nonresearch sites. However, it could increase the risk of spillover, if control group cases were a smaller proportion of each worker's caseload.
- Research sites could be located in contiguous counties, since migration might be more likely to nearby counties than other counties. However, this could create trade-offs with goals of external validity, since it may be difficult to choose representative sites that are also contiguous.
- Statewide administrative records could append original experimental/control status information to individual records as well as case records. This would make it easier to identify, at the time of application or redetermination, individuals who have split off from research cases or who are merging with other cases and make it less likely that individuals' experimental/control status will be changed as the result of administrative error or manipulation.